I really loved this book, probably because Lee Scoresby and his daemon Hester are some of the most memorable characters in His Dark Materials. After the general feeling of disappointment Lyra's Oxford generated, I think fans will be pleased with this short and pungent cowboy fantasy. Our hero arrives on the scene with a crash of his balloon, and the story doesn't slow down until the final showdown (its a cowboy story, so there has to be a showdown).
Iorek the Polar Bear makes a guest appearance, but not for very long, so don't read it expecting to get a lot of bear action.
I would give the book a hearty Yay, but it probably doesn't stand alone terribly well given that a person really needs to know what a daemon is, and the appearance of Iorek would seem rather abrupt if the reader weren't expecting it.
So, Nay, but it you love a good western, its not to be missed!